A narrow directional condenser microphone having an acoustic tube is also called a line microphone or a gun microphone from its shape, and is configured so that in the cylindrical acoustic tube, a similarly cylindrical condenser microphone unit is housed.
For example, as described in Japanese Patent No. 256295, the narrow directional condenser microphone is configured so that a diaphragm included in the condenser microphone unit is arranged perpendicularly to the tube axis (the directional axis or the sound pickup axis) of the acoustic tube.
Especially when sounds are picked up from a distant sound source, the narrow directional condenser microphone of this type is required to generate little self noise because the sound waves delivered from the sound source are weak.
In order to meet this requirement, the effective diaphragm area of the diaphragm included in the condenser microphone unit has only to be increased. For the construction of the conventional example, however, the diameter of the condenser microphone unit is restricted by the inside diameter of the acoustic tube, and the increase in the effective diaphragm area of the diaphragm has its limits.
If the acoustic tube having a large inside diameter is used, accordingly a unit having a large diameter can be adopted. However, a large-diameter acoustic tube unfavorably not only impairs the appearance of microphone but also increases the weight thereof.
As one method for increasing the effective diaphragm area of the diaphragm, Japanese Patent No. 3325913 describes that the diaphragm is made to have a rectangular shape. In this specification, the condenser microphone unit having a rectangular diaphragm is sometimes referred to as a rectangular unit.
Besides, as the microphone using the rectangular unit, Japanese Patent No. 4383242 describes that a narrow directional microphone is provided by using three rectangular units. Also, Japanese Patent Application Publication No. 2002-78062 describes a double capsule microphone having two rectangular units.
Although various types of microphones using the rectangular unit have been proposed, attempts have not been made to make the microphone have a special shape matching the rectangular unit and to incorporate the rectangular unit in the cylindrical acoustic tube.
Accordingly, an object of the present invention is to provide a narrow directional condenser microphone having an acoustic tube, in which a condenser microphone unit having a large effective diaphragm area is arranged on the rear end portion side of the acoustic tube to achieve high sensitivity without an increase in the diameter of the acoustic tube.